The light is too dark even at a high value
There are a few possibilities why this could be happening, unfortunately the question does not have enough information to be able to determine the cause for sure. So here are a few things to look for:


The light is blocked by other objects or is not reaching the featured object

Check that there are no hidden or disabled objects blocking the path of the lights.
The lamps could be disabled for rendering in the outliner

To fix enable the camera icon.


The lights might be in a different layer that is not being rendered.



To fix move them to the active layer or enable the layer for the lights as part of the rendered layers.
The overall birghness of the scene could be clamped in the render settings.



Any value below 1 in Clamp Direct, will make the image darker. Clamping is really preventing the ligthts to be any brighter than the value specified.

To fix, disable clamping by resetting the values back to 0 (0 = no clamping)
The model might be too large or the lights might be too far away so the model is lit just by the color of the environment.

Cycles uses values close to those of the real world. If your model is set at a scale where the turntable is as large as a city block, then one small light will definitely not be enough, or it would need to be very bright. As a rule it is better to try to keep scenes to real world sizes.